This invention relates to improvements in filters for use in the filtration of blood such as in cardiopulmonary bypass system used during open heart surgery. U.S. Pat. No. 3,701,433 discloses a disposable blood filter employing a woven mesh having a pore size within the range of 25 to 50 microns for removal of, inter alia, microemboli from artificially oxygenated blood prior to reintroduction into a patient during a cardiopulmonary bypass. Such filters marketed by Pall Corporation, the present assignee, and similar filters marketed by numerous others have proven highly effective and beneficial and are now universally employed during surgery involving cardiopulmonary bypass.
For well known medical reasons which need not be discussed here, it is absolutely critical that no free gas, whether in the form of microemboli or gross bubbles, be present in the blood returned to a patient. While most commercial filters are fairly effective in removing microemboli from an otherwise steady flow of blood, none of the commercial filters presently on the market can automatically assure that gross amounts of air presented to the upstream side of the filter element will not pass through the element and into the patient's blood stream. While a fine pore filter element fully wetted with blood is effective to prevent passage of a gas at a given limited rate, this effectiveness decreases dramatically when this rate is exceeded. Accordingly, one operating protocol requires a technician to continually monitor the blood filter, prepared to take emergency measure to assure that air does not accumulate upstream of the filter element and expose the filter to the gas rather than blood. Most filters of the type disclosed in U.S. Pat. No. 3,701,433 and used commercially include a port which may be employed to vent the gas but the port size typically is such that it is inadequate to vent a 100 per cent gas flow as when the blood feed to the pump is interrupted. With such an occurrence, at a flow rate of six liters per minute, the exterior chamber of a blood filter of the type shown in FIG. 4 of U.S. Pat. No. 3,701,433 will fill with gas in approximately two seconds and, shortly thereafter, air is likely to pass through the filter element and into a patient. Accordingly, a filter which quickly, automatically and safely vents all of the air presented at the upstream chamber, even in the event of a catastrophic failure, is highly desirable both in terms of safety to the patient and economy in use of resources and personnel.